Field of Technology
The present disclosure relates to a display device, and more particularly, to a display device that calibrates the sensing circuit for sensing the characteristics of a display panel in real-time.
Discussion of the Related Art
An active matrix type organic light emitting display covers an organic light emitting diode (hereinafter, referred to as “OLED”) which emits light by itself, and has advantages of a fast response speed, high light emitting efficiency, high brightness, and a wide viewing angle.
An OLED that emits light by itself includes an anode electrode, a cathode electrode, and organic compound layers formed therebetween. The organic compound layers include a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the HTL and electrons passing through the ETL are transferred to the EML to form excitons. As a result, the light emitting layer EML generates visible light.
In an organic light emitting diode display device, pixels each including an OLED are arranged in a matrix form, and luminance is controlled by controlling the amount of emitted light of the OLED according to the gradation of image data. Each of the pixels includes a driving element, i.e., a driving thin film transistor TFT, which controls the pixel current flowing the OLED according to the voltage applied between its gate electrode and the source electrode. The electrical characteristics of the OLED and the driving TFT deteriorate with time and may cause a difference in the pixels. Electrical deviations between these pixels are a major factor in degrading image quality.
The external compensation technology is known that measures the sensing information corresponding to the electrical characteristics of the pixels (the threshold voltage and the electron mobility of the driving TFT and the threshold voltage of the OLED) and modulates image data in an external circuit based on the sensing information, in order to compensate for the electrical characteristic deviation between the pixels.
In this external compensation technology, the electrical characteristics of pixels are sensed by using a sensing block embedded in a source drive IC (integrated circuit). The sensing block which receives pixel characteristic signals in the form of a current comprises a plurality of sensing units including a current integrator and a sample/hold unit, and an analog-to-digital converter ADC. The current integrator performs an integration of a pixel current input through a sensing channel to produce a sensed voltage. This sensed voltage is passed to the ADC through the sample/hold unit, and converted to digital sensing data by the ADC. A timing controller calculates a pixel compensation value for compensating for variations in the electrical characteristics of pixels based on the digital sensing data from the ADC, and corrects the input image data based on the pixel compensation value.
Since the organic light-emitting display comprises a plurality of source driver ICs for driving the display panel on an area basis in a segmented fashion, sensing blocks, each embedded in each source drive IC, sense the pixels on the display panel area by area in a segmented fashion. When pixels are sensed in a segmented fashion by the sensing blocks, sensing accuracy may be low due to offset variations between the sensing blocks. Especially, the ADC inside the source drive IC changes in its characteristics depending on a temperature or surrounding environments, so the output of the ADC maintains a constant value to some degree at a certain range of a room temperature, but at a high temperature outside the room temperature it changes to a value significantly different from that at the room temperature. This output characteristics of the ADC affect the pixel sensing data for the panel, causing a block dim phenomenon in which a difference in luminance is displayed between the areas where the source drive ICs are responsible when displaying an image.
To solve the block dim phenomenon, an offset deviation among the sensing blocks should be compensated through a calibration process. The calibration process applies a test current to each sensing block to obtain the sensing data for calibration and calculates the compensation values for calibration which can compensate for the offset deviation between the sensing blocks based on the sensing data for calibration. A timing controller increases compensation accuracy by referring to the compensation values for calibration as well as the compensation values for pixel when adjusting input image data.
FIGS. 1 and 2 show the configurations implementing conventional calibration operations.
The first conventional calibration method in FIG. 1 provides one common current source Ix in order to apply a test current to, for example, 3 sensing blocks equipped in 3 source drive ICs SIC1, SIC2 and SIC3. This calibration method sequentially applies the test current to the 3 sensing blocks while alternatively turning on the switches SW1, SW2 and SW3 connected between the common current source Ix and the source drive ICs SIC1, SIC2 and SIC3.
The second conventional calibration method in FIG. 2 provides 3 separate current sources I1, I2 and I3 in order to apply a test current to for example 3 sensing blocks equipped in 3 source drive ICs SIC1, SIC2 and SIC3. This calibration method simultaneously applies the test current to the 3 sensing blocks via the separate current sources I1, I2 and I3.
The first calibration method does not cause a calibration error by the deviation of the current sources because the common current source Ix is used, but has the problem of increasing a tack time because all source drive ICs SIC1, SIC2 and SIC3 are sequentially calibrated via one common current source Ix.
The second calibration method has the advantage of decreasing the tack time because the source drive ICs SIC1, SIC 2 and SIC3 are simultaneously calibrated via separate current sources I1, I2 and I3, but has the problem of causing the calibration error due to the deviation among the separate current sources I1, I2 and I3. In case that a number of source drive ICs are used in a display device, for example 20 source drive ICs are used, the number of separate current sources should be 20, and the circuit configuration of the current sources should become precise and complicated or a circuit scale would become large so that respective current sources can output the currents having a very small deviation and almost a same value.
Meanwhile, because an offset deviation occurs between sensing units corresponding to respective sensing channels, when calibration operations are performed for respective sensing units in order to calibrate the deviation of the sensing units, both of the first and second calibration methods have the problem of increasing the tack time. Especially, in the case of the first calibration method, the problem that the time required for the calibration operation is increased is the most serious.